Henderson Hasselbalch Calculator
Use the Henderson Hasselbalch equation to find the pH of a buffer. Enter the pKa and the concentrations of the conjugate base and weak acid to get the pH, or enter a target pH and pKa to find the base to acid ratio you need.
How to use the Henderson Hasselbalch equation
- Enter the pKa of the weak acid.
- Enter both the conjugate base and acid concentrations, or enter a target pH.
- Read the pH and the base to acid ratio, from pH = pKa + log10([A-]/[HA]).
Examples
Equal concentrations
pKa 4.76, [A-] = [HA]
pH 4.76, ratio 1
Tenfold base to acid
pKa 4.76, [A-] 1, [HA] 0.1
pH 5.76, ratio 10
Frequently asked questions
What is the Henderson Hasselbalch equation?
It relates buffer pH to pKa: pH equals pKa plus the base-10 log of the ratio of conjugate base to weak acid, pH = pKa + log10([A-]/[HA]).
When does pH equal pKa?
When the conjugate base and weak acid concentrations are equal, the ratio is 1 and log10(1) is 0, so the buffer pH equals the pKa.
How do I find the ratio from a target pH?
Rearrange to ratio equals 10 to the power of pH minus pKa. Enter a pH and pKa and the tool returns the base to acid ratio you need.
Does the equation work for any buffer?
It works well for buffers within about one pH unit of the pKa. Far from the pKa, or at very low concentrations, the approximation loses accuracy.
Do I use molarity for the concentrations?
Yes, use mol/L, though only the ratio matters. Equal units cancel, so any consistent concentration unit gives the same pH.
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